Ophthalmic glass



United States Patent 3,178,274 OPHTHALMIC GLASS James E. Duncan, NatronaHeights, and Samuel L. Seymour, Oalrrnont, Pa, assignors to PittsburghPlate Giass Company, Pittsburgh, Pa, a corporation of Pennsylvania NoDrawing. Original appiication May 13,158, Ser. No. 734,847, new PatentNo. 3,920,803, dated Feb. 13, 1962. Divided and this appiication May 25,1961, Ser.

3 Claims. or. 65-39 This invention relates to an ophthalmic glass and ithas particular relation to a glass having a relatively high softeningpoint and a high index of refraction so as to be useful as the segmentin a particular method of forming a multifocal, ophthalmic lens.

A multifocal lens is conventionally composed of two different ophthalmicglasses. A major portion of the lens is composed of a crown glass orother glass which has a relatively low index of refraction and which ishighly refined and free from seeds, striae and other imperfections whichwould impair the optical properties of the lens. The crown glass has aground and polished recess in it and a glass in the form of a smallsegment is fused in the recess. The segment glass has a higher index ofrefraction than the crown glass and it is the portion of the fusedmultifocal lens which is used for reading.

One example of a crown glass is an alkali-lime silica glass containingapproximately 70 percent SiO 8.5 percent Na O, 7.5 percent K 0, 12percent C210 and 2 percent refining agents such as Sb O The segment isusually made from highly refined glasses known as barium, flint,barium-flint or barium crown glasses. Examples of these glasses areshown in US. Patents Nos. 2,523,264, 2,523,255, 2,523,266 and 2,528,634.The crown glass and the segment glass preferably have substantially thesame coefiicient of thermal expansion. The barium type glasses usuallyhave a slightly lower softening point than the crown glasses and theflint glasses usually have a softening point considerably lower than thecrown glasses.

In the manufacture of a multifocal lens, a processed, segment blank isfused to a processed major blank. The major blank and the segment blankare made in a great variety of shapes and sizes by the glassmanufacturers. These blanks are semi-finished by the various lensprocessing companies and are usually assembled in the manner hereinafterdescribed. The face of the segment blank to be engaged by the majorblank is ground and polished to stock minor focal fields usuallyexpressed in diopters. A suitable polished depression or countersink isformed in the major blank to receive the segment. The segment is placedin the depression and the assembly is heated to fuse the segment glassto the glass used for the major blank. Thereafter, the fused bifocalblank is ground and polished on both sides to form a semi-finished lens.

The perimeter of the segment blank is not always of circular form.Frequently, it is in the form of a semicircle, rectangle or modificationthereof. The various shapes make it difficult to form correspondinglyshaped segment receiving depressions in the major blank. One usualpractice is to grind and polish in the major blank a circular depressionwhich is large enough to receive the segment. Any remaining portion ofthe depression surrounding the segment is filled with smaller shapedportions of crown glass which are fused to the segment prior to fusionof the segment in the depression. The whole assembly is then fusedtogether so that only the segment is visible apart from the majorportion of the lens when the lens is ground and polished.

When assembling the segment in the depression of the major portion ofthe lens, great care must be exercised auaari Patented Apr. 13, 1965 tomaintain the face of the depression and the engaging face of the segmentfree from lint, dust, finger marks and other foreign matter. Likewise,great care must also be exercised to avoid entrapping air between thesegment face and the bottom of the depression. One general practice isto make the face of the segment of a slightly different curvature thanthe depression so that upon heating to fuse the major portion of thelens and segment together, the segment glass will flow into the exactcurvature of the depression and the entrapped air will escape duringthis heating. Another general practice is to assemble the major portionof the lens and the segment with one side of the segment slightlyelevated by the introduction of a small glass or wire wedge between thesegment and the major .portion of the lens. The assembly is heated to atemperature such that the segment glass softens, settles and fuseswithin the depression and in so doing, pushes the air out of theinterspace between the elements, the fusing taking place gradually fromone side of the depression to the other. All of these methods involvethe use of the conventional crown and segment glasses mentioned above.

A novel method of forming a multifocal lens blank is disclosed in anapplication of Samuel L. Seymour, Serial No. 688,560, filed October 7,1957. In this process, a mold is used which conforms to the shape of themultifocal lens blank and which has a recess or countersink ofconfiguration which is complementary to the segment. The segment, havingan optically finished surface on one side thereof, is preheated to atemperature below its softening point and placed within the recess orcountersink with its finished surface facing upwardly. Molten glass ofoptical quality to form the major portion of the multifocal lens blankis then fed from a glass furnace discharge orifice onto the mold surfacein such a manner that the molten glass does not contact the preformedsegment but has a part of its perimeter in close proximity thereto, Amolten mass of glass is allowed to accumulate in the mold and the feedis stopped. Immediately thereafter, the accumulated mass of molten glassis laterally pushed so as to flow over the mold surface and the finishedsurface of the heated segment. In so doing, the molten glass fuses tothe segment without the entrapment of air at the interface or area offusion. Subsequently, if necessary or desirable, the distributed mass,while still in its molten condition, may be subjected to a verticallydirected pressing with a suitably shaped molding member to furthercomplete the shaping of the composite multifocal lens blank.

This novel method of manufacture of a multifocal lens blank requires theuse of a segment glass having a softening point which is much higherthan that of the high index, segment glasses which are used in theconventional processes described above. The segment glass which isemployed in the novel method should have a softening point which ishigher, for example, to F. or more than the softening point of the glasswhich forms the major portion of the multifocal lens in order to avoiddistortion of the finished curved surface of the segment during fusingof the molten major portion glass to the segment.

The present invention is concerned with providing a glass for use as thesegment glass in the novel method of manufacture of multifocal lensblanks as described above, which glass has a relatively high softeningpoint, i.e., from 1385 F. to 1475 F., an index of refraction, N between1.57 and 1.67, a high reciprocal dispersion ranging from 40 to 56 and acoefficient of expansion which is compatible with the glass of the majorportion of the lens within the range of 7 to 10X 10 per C. between 25 C.and 300 C. Novel glasses within the purview of the invention areprepared by the use of conventional glass making materials inconventional, ophthalmic glass manufacturing equipment. The followingbatches in pounds and compositions in calculated percent by weightproduced therefrom are exemplary of the glasses of the invention:

4 the temperature of the furnace is gradually lowered in about A. of anhour to about 2075 F. A furnace temperature of about 2075 F. is thenmaintained for a period of about /2 hour. The pot of glass is thenremoved from BATCHES Pounds Ingredients Sand 956 968 968 920 956 1, 135956 780 788 788 715 788 716 Barimn Carbonate" 908 939 939 939 908 900908 908 939 877 877 39 939 Sodium Carbonate 204 174 174 174 174 200 263277 298 277 277 277 Potassium Carbonate 130 107 107 107 130 280 130 1212 12 12 Calcium Carbonate. 151 237 151 151 157 268 86 237 237 ZincOxide 80 80 213 128 80 122 58 48 48 48 167 167 109 80 32 107 26 41 41 4126 26 112 88 88 88 88 88 255 218 218 218 255 107 255 403 403 403 477 403403 Sodium Nitrate 30 100 100 100 15 100 100 100 100 100 100 AntimonyTrioxide 15 15 Arsenic Trioxide.'. 15 15 15 15 15 15 15 15 15 15Aluminum Hydrate 110 Calcium Fluoride Sodium Silico Fluoride 24Composition Component referred range 1 2 3 4 5 6 7 8 9 10 11 12 13 Si043. 9 43. 6 43. 6 41.6 44. 3 36. 8 38. 8 35. 5 -52 BaOU 29. 5 30. 5 30.5 30. 5 29. 8 28. 5 30. 5 30.3 28. 1-34 5. 5 5. 8 5. 8 5. 8 4. 5 8. 38.3 8. 3 0-12 3.6 3.0 3.0 3.0 3.7 .3 .3 .3 0-10 3. 5 5. 5 3.5 3. 6 5.65. 6 0-10 3. 4 3. 4 8. 9 5. 4 3. 4 0-10 2.0 2.0 3.3 1.3 7.0 0-10 7.0 5.9 5. 9 5. 9 7. 0 12. 9 10. 9 10.9 2-15 1.1 1. 7 1. 7 1.7 1.1 3. 7 3. 73. 7 0-5 .6 .6 .6 .6 .6 .6 0-2 1. 0 0-2 0-1. 5

3.0 0-5 Index of Refraction N 1. 616 1. 655 1. 645 1. 660 1. 57-1. 67Coeff. of expansion X 10- per C.

between 25. C. and 300 C 9.0 9. 2 8. 6 8. 7 8. 5 8 6 8. 6 8 4 7. 0-10. 1Softening point F. (temp. at which the log of the vis. in poiscs is 101, 389 1, 385 1, 385 1, 395 1, 422 1, 444 1, 445 1, 437 1, 385-1, 475Reciprocal Dispersion (N 1)/ Nit-N; 49. 8 50. 7 50. 2 50. 1 50. 5 45. 246.1 42. 6 40-56 The softening point recited in the table is defined asthe temperature in degrees Fahrenheit at which the viscosity of theglass is 10 poises. This viscosity is obtained according to the methoddescribed on pages 228 to 231 of the 1957 supplement to the Book of ASTMStandards, a publication of the American Society for Testing Materials,copyright 1958.

In the preparation of the glasses, the batch ingredients are thoroughlymixed in the proportions necessary to produce the glasses. Various sizepots or crucibles may be employed and the temperatures and times willvary according to the amount of glass being formed. The temperatures andmelting conditions herein recited are employed to make 85 to 100 poundsof these glasses in clay pots in a furnace heated by the controlledcombustion of natural gas.

The empty pot is preheated in the furnace at a furnace temperature ofabout 2200 F. A portion of the mixed batch is ladled into the preheatedpot and the furnace temperature is gradually increased. The remainingportion of the mixed batch is ladled into the pot at a. uniform rateover a period of 3% hours and the temperature is raised gradually duringthis period to about 2600 F. During the next /2 hour the furnacetemperature is further increased to between 2650 and 2670 F. and withinthis time substantially all of the glass making materials are melted.This temperature is maintained for the following 1/; hours during whichtime the chemical reactions are completed and the glass becomessubstantially free of bubbles. During the melting and high temperaturereacting periods just described a neutral or slightly oxidizingatmosphere is maintained Within the furnace.

After the glass has become substantially free of bubbles,

the furnace, the glass is poured on a metal table and is rolled into theform of a sheet. The sheet is placed in a kiln and cooled from 1050 F.to 850 F. at a rate of about 5 F. per minute. Thereafter, it is cooledmore rapidly to room temperature and cut into pieces suitable forpreliminary property tests.

This glass may be further refined to improve its optical quality. Theglass is broken into cullet and about 15 pounds is placed in a platinumlined clay crucible which has been preheated to a temperature of about2000 F. The crucible and contents are placed in an electrically heatedfurnace which is at a temperature of about 2200 F. The furnacetemperature is gradually raised at a uniform rate to about 2670 F. overa period of about 2% hours. This temperature is maintained for a periodof about 1% hours during which time the bubbles are substantiallyeliminated from the melt.

At the end of this period, a platinum stirrer is inserted in the moltenglass. The glass is stirred while the temperature of the furnace isgradually reduced over a period of hour to a temperature of 2075 F. Afurnace temperature of 2075 F. is maintained for a period of /2 hourWhile stirring is continued. The stirrer is then removed from the glassand the crucible is removed from the furnace. The glass is poured on ametal table and rolled into the form of a sheet. The sheet is placed ina kiln and cooled from 1050 to 850 F. at a rate of about 5 F. perminute. Thereafter it is cooled more rapidly to room temperature and cutinto pieces suitable for processing according to conventional techniquesinto segments.

In the manufacture of the glasses of the invention, silica is theprincipal glass former. If lower than 35 percent by weight of SiO isemployed in the composition, it tends to reduce the chemical durabilityof the glass, whereas amounts higher than 52 percent by weight of S makeit increasingly difficult to achieve a glass having a high index ofrefraction.

The total of the alkali metal oxides, Na O and K 0, is maintainedbetween 6 and 12 percent by weight. A minimum amount of alkali metaloxide is required to achieve the desired coefiicient of expansion, butan excess is to be avoided for it has a deleterious effect on thechemical durability of the glass and it also lowers the softening point.It is desired that no lithia be present in the glasses of the invention,for lithia greatly lowers the softening point of the glasses.

A relatively large amount of barium oxide is desired to achieve the highindex of refraction, but too much barium oxide causes the glasses todevitrify. Careful selection and control of the other ingredients of theglasses permits the use of the high amount of barium oxide to obtain theunusually high softening point and prevent devitrification of theglasses of the invention. Calcium oxide, zinc oxide and lead oxide arepresent to provide the required properties and prevent devitrification.Cadmium oxide and strontium oxide may also be used for this purpose. Anyone or combination of the oxides of calcium, zinc, lead, cadmium andstrontium in an amount between 5 to 15 percent by weight is suitable.

Zirconium dioxide is necessary to produce a glass having a highsoftening point; however, an excess of Zirconium dioxide undesirablyreduces the coefficient of expansion of the glass. About 0.5 to 5percent by weight of titanium dioxide may be used to control therefractive index of the glass. Its use is restricted to less than about5 percent because it lowers the reciprocal dispersion unduly. No boronoxide is present in the glasses of the invention. It has been found thata combination of B 0 and the relatively high amount of BaO present inthe glasses of the invention adversely affects their chemicaldurability.

Approximately 95 or more percent by Weight of the glasses which are thesubject of this invention is comprised of SiO BaO, ZrO Na O, K 0 and oneor more bivalent metal oxides selected from the group consisting of CaO,ZnO, PbO, CdO and SrO. The remaining approximately 5 percent or less byweight of the glasses may be made up of TiO fining agents, melting aidsand other materials such as colorants which may affect the transmission,absorption or other properties of the glasses without unduly loweringtheir softening point or harmfully affecting their other desirableoptical properties.

For example, fluorine may be present to act as an aid in refining theglass at high temperatures. Its use is restricted to less than about 1.5percent by weight because when used in larger amounts objectionableopalescence forms in the glass due to precipitation of fluoride crystalsin the body of the glass. Aluminum oxide may also be included in theglasses to help obtain the high softening point. Its use is limited toless than about 5 percent by weight because higher amounts have atendency to cause the glasses to devitrify. The oxides of antimony andarsenic are added as conventional fining agents and other oxides orother compounds which act by themselves or collectively as fining agentsmay be employed in the practice of the invention.

The glasses of the invention may be utilized as segments in the novelprocess described in the previously mentioned application of Samuel L.Seymour in combination with conventional crown glasses and preferablywith special low softening point crown glasses containing 62 to 66percent SiO 0 to 15 percent Na O, 0 to 15 percent K 0, the sum total ofalkali metal oxides being 12 to 17 percent, 15.2 to 20 percent P00, 0.5to 4 percent TiO and 0.5 to 3 percent A1 0 as disclosed in anothercopending application of Samuel L. Seymour, Serial No. 538,516, filedOctober 4, 1955. The composition of an example of such a glass is 65.0percent SiO 5.3 percent Na O, 9.3 percent K 0, 18.6 percent PbO, 0.5percent TiO 0.5 percent A1 0 and 0.8 percent Sb O These crown glasseshave a coefiicient of expansion between about 7 to 10x10- per C. between25 and 300 C., an index of refraction between 1.520 and 1.540 and asoftening point of about 1125 to 1250 F. The softening point of thesegment glass should be at lea-st F. or more above the softening pointof the crown glass and it is preferred that the softening point of thesegment glass be from to 350 F. or more above the softening point of thecrown glass when utilized in the novel process.

In the manufacture of the multifocal lens blanks according to the novelprocess, the blank mold is heated to a temperature of about 700 to 800F. A segment glass having a composition as set forth in Glass No. 1 ofthe table and having an optically finished surface is preheated to atemperature of about 1250 to 1300" F. and placed within a recess orcountersink within the heated mold with its finished surface facingupwardly. Molten crown glass of optical quality made according toconventional continuous optical glass manufacturing techniques issimultaneously deposited onto the mold surface in such a manner that themolten glass does not contact the preformed segment, but has a part ofits perimeter in close proximity thereto. This glass may have acomposition the same as that of the exemplary glass set forth in thepreceding paragraph. A molten mass of glass is allowed to accumulate inthe mold and the feed is then stopped. The temperature of the moltenglass is about 1875 to 1975 F. Immediately there after the accumulatedmass of molten glass is laterally pushed by a molding member heated to atemperature of about 700 to 800 P. so as to flow the glass over the moldsurface and the finished surface of the minor segment and to fuse to thesegment without the entrapment of air at the interface or area offusion. The molding member is then vertically impressed on the moltenglass to further shape the multifocal lens blank. The blank is removedfrom the mold and cooled slowly in an annealing lehr from a temperatureof 1050 to 850 F. at a rate of 5 F. per minute. The multifocal lensblank may then be ground and polished according to conventionalophthalmic procedures to a finished lens suitable for incorporation inspectacles.

Although the present invention has been described with reference tospecific details of certain embodiments thereof, it is not intended thatsuch details shall be regarded as limitations upon the scope of theinvention except insofar as set forth in the following claims. Thisapplication is a continuation-in-part of our copending applicationSerial No. 478,518, filed December 29, 1954, now abandoned, and is adivision of our copending application Serial No. 734,847, filed May 13,1958, now US. Patent 3,020,803.

We claim:

1. In the method of forming a multifocal lens blank which comprisesplacing a segment having an optically finished surface in a mold withthe optically finished surface facing upwardly, depositing a molten massof a crown glass onto the mold surface in such a manner that the moltenglass does not contact the segment glass and laterally pushing themolten mass of crow glass over the mold surface and the finished surfaceof the segment to fuse the molten glass to the segment glass without theentrapment of air at the interface or area of fusion, the softeningpoint of the segment glass being at least 100 F. higher than thesoftening point of the crown glass, the improvement which comprisesusing as the segment glass a boron-free and lithia-free glass having anindex of refraction, N between 1.57 and 1.67, a softening point above1385 F. and a coefiicient of thermal expansion of 7 to 10 :1O- per C.between 25 and 300 C. which consist essentially of the followingingredients in percent by weight: 35 to 52 percent SiO 0 to 12 percentNa O, 0 to 10 percent K 0,

7 the sum of Na O and K being 6 to 12 percent, 28.1 to 34 percent BaO, 2to 15 percent ZrO 0 to 10 percent CaO, 0 to 10 percent ZnO and 0 to 10percent PbO, the sum of CaO, ZnO and PhD being 5 to 15 percent, theabove listed essential ingredients constituting at least 95 percent byweight of the glass.

2. A boron-free and lithia-free glass having an index of refraction, Nbetween 1.57 and 1.67, a softening point of at least 1385 E. and acoefiicient of thermal expansion of 7 to 10 :10 per C. between 25 and300 C. which consists essentially of the following in gredients inpercent by weight: 35 to 52 percent SiO;, 0 to 12 percent Na O, O to 10percent K 0, the sum of Na O and K 0 being 6 to 12 percent, 28.1 to 34percent B210, 2 to 15 percent ZrO 0 to 10 percent CaO, 0 to 10percent-Z110 and 0 to 10 percent PbO, the sum of CaO, ZnO and PbO being5 to 15 percent.

3. A boron-free and lithia-free glass having an index of refraction, Nbetween 1.57 and 1.67, a softening point of at least 1385 F. and acoeflicient of thermal expansion of 7 to 10 l0- per C. between and 300C. which consists essentially of the following ingredients in percent byweight: to 52 percent SiO 8 0 to 12 percent Na O, 0 to 10 percent K 0,the sum of Na O and K 0 being 6 to 12 percent, 28.1 to 34 percent BaO, 2to 15 percent ZrO 0.5 to 5 percent TiO 0 to 10 percent CaO, 0 to 10percent ZnO and 0 to 10 percent PbO, the sum of CaO, ZnO and P190 being5 to 15 percent.

References Cited by the Examiner UNITED STATES PATENTS 2,391,468 12/45Long 1l7129 2,433,013 12/47 Ziegler 49-821 2,491,965 12/49 Ganci 106-48X 2,523,264 9/50 Armistead l06-53 2,523,265 9/50 Armistead 106532,660,531 11/53 Fraser 106-48 2,699,399 l/55 Armistead 10653 FOREIGNPATENTS 837,727 6/60 Great Britain.

TOBIAS E. LEVOW, Primary Examiner. JOSEPH REBOLD, Examiner.

1. IN THE METHOD OF FORMING A MULTIFOCAL LENS BLANK WHICH COMPRISESPLACING A SEGMENT HAVING AN OPTICALLY FINISHED SURFACE IN A MOLD WITHTHE OPTICALLY FINISHED SURFACE FACING UPWARDLY, DEPOSITING A MOLTEN MASSOF A CROWN GLASS ONTO THE MOLD SURFACE IN SUCH A MANNER THAT THE MOLTENGLASS DOES NOT CONTACT THE SEGMENT GLASS AND LATERALLY PUSHING THEMOLTEN MASS OF CROW GLASS OVER THE MOLD SURFACE AND THE FINISHED SURFACEOF THE SEGMENT TO FUSE THE MOLTEN GLASS TO THE SEGMENT GLASS WITHOUT THEENTRAPMENT OF AIR AT THE INTERFACE OR AREA OF FUSION, THE SOFTENINGPOINT OF THE SEGMENT GLASS BEING AT LEAST 100*F. HIGHER THAN THESOFTENING POINT OF THE CROWN GLASS A BORON-FREE AND LITHIA-FRE GLASSHAVING AN INDEX OF REFRACTION, ND, BETWEEN 1.57 AND 1.67, A SOFTENINGPOINT ABOVE 1385*F. AND A COEFFICIENT OF THERMAL EXPANSION OF 7 TO10X10**-6 PER *C. BETWEEN 25 AND 300*C. WHICH CONSIST ESSENTIALLY OF THEFOLLOWING INGREDIENTS IN PERCENT BY WEIGHT: 35 TO 52 PERCENT SIO2, 0 TO12 PERCENT NA2O, 0 TO 10 PERCENT K2O, THE SUM OF NA2O AND K2O BEING 6 TO12 PERCENT, 28.1 TO 34 PERCENT BAO, 2 TO 15 PERCENT ZRO2, 0 TO 10PERCENT PBO, THE SUM OF CAO, ZNO AND PBO BEING 5 TO 15 PERCENT, THEABOVE LISTED ESSENTIAL INGREDIENTS CONSTITUTING AT LEAST 95 PERCENT BYWEIGHT OF THE GLASS.